Sensorineural hearing loss (SNHL) is common, and its impact on human communication and quality of life, significant. It is estimated that some 28 million individuals in the United States suffer from hearing loss. As our population ages, hearing loss prevalence is expected to climb rapidly, nearly doubling by the year 2030. Causes range from degenerative processes associated with aging and genetic disorders to environmental exposure to loud sounds and toxic agents. Consequences range from moderate communication difficulty and social withdrawal to profound deafness and its significant challenges. At present, management of SNHL centers on the use of hearing aids and cochlear implants. However, such treatments cannot address hearing loss prevention, they cannot minimize hearing loss progression and, even with optimal device fitting, cannot increase a damaged ear's basic capacity. As a result, many users continue to experience significant communication difficulties.
Recent advances in the pharmacology and molecular biology of hearing have revealed new and powerful possibilities for preventing or minimizing hearing loss. The crux of the problem in SNHL is loss of the delicate cochlear sensory cells that detect the exquisitely small mechanical vibrations associated with sound. In human ears, once lost or damaged, these sensory cells do not regenerate and this compromise is often followed by secondary degeneration of auditory neurons. However, scientists and clinicians are making rapid progress in understanding the molecular mechanisms associated with cochlear and auditory nerve degenerative processes. Additional insight into the molecular signals involved in generating new hair cells is rapidly accumulating, and with this insight comes the promise of novel and precise drug treatments. Moreover, the extraordinary progress that has been made in defining the genes involved in a number of human genetic forms of deafness offers hope for gene-transfer and molecular approaches to treat these diseases.
For therapies based on these discoveries to become clinically useful, it will be necessary to develop safe and reliable mechanisms for the delivery of complex compounds into the inner ear. Direct delivery to the fluids of the inner ear is necessary because of the presence of a blood-labyrinth drug barrier, which is anatomically and functionally similar to the blood-brain barrier. That is, through the presence of so-called ‘tight junctions’ between adjacent cells in the inner ear end organs, substances outside these organs encounter a substantial physical barrier to entry, this protecting the delicate sensory structures within from insult. This ‘protection’, however, also prevents certain molecules with potentially therapeutic effect from gaining access to their inner ear targets. Prime candidates for exclusion from the cochlea after systemic injection are complex molecules, such as proteins and peptides, as well as any molecule that is not lipid-soluble.
Current otologic practice requires drug delivery to the inner ear, but uses inefficient routes. Drugs are commonly delivered systemically, with the hope that they will find their way to their intended inner ear targets in the form and concentration desired and without serous side effects. Systemic corticosteroids, for example, are used in the otologic management of idiopathic sudden and immune-mediated sensorineural hearing losses. Their clinical usefulness, however, is limited by undesirable side effects arising from the high systemic doses required to achieve sufficient cochlear fluid levels of drug to produce the intended inner ear effects.
Local drug application by transtympanic perfusion of the middle ear with the goal of diffusion through the round window membrane (RWM) into the fluid spaces on the inner ear) was introduced nearly 50 years ago with aminoglycoside treatment of Meniere's disease. This method or some variant remains in common use in the treatment of inner ear diseases, notably the intractable vertigo that can be associated with Meniere's disease, but has been used as well for sudden sensorineural hearing loss, autoimmune inner ear disease, and even tinnitus. Accomplished as an office procedure, a drug is injected through the tympanic membrane into the middle ear space. The patient then lies with the treated ear ‘up’ so that the drug has a better chance of making contact with the RWM, through which the drug must diffuse to gain access to the inner ear. With the goal of extending the time of drug availability to the inner ear, newer methods of intratympanic drug delivery have employed several strategies to prolong drug contact with the RWM, including, placing absorbent material on or near the TWM and using pump-driven microcatheter systems.
Delivery of drugs to the middle ear reduces systemic side effects, but access to the inner ear is unpredictable. Middle ear application has advantages over systemic drug delivery, in that drugs so applied can reach their desired targets at higher concentrations and without unwanted systemic side effects. The application is straightforward, and complications are minimal. A major limitation of these methods, however, is the inability to precisely control the amount of drug that diffuses from the middle ear through the RWM into the inner ear. Individual variation in mucous membrane thickness, mucosal folds and middle ear anatomy can have a significant impact on the amount of drug that ultimately enters the inner ear. Some commentators, for example, report round window niche obstruction in 33% of human ears. This becomes even more problematic when considering delivery of coplex macromolecules with limited diffusion coefficients and those requiring sequenced delivery. Additionally, the bolus application used by certain existing systems makes them poorly suited for direct inner ear delivery. Although such devices may be useful for delivery of low molecular weight, stable, lipid-soluble compounds like steroids, they would not be suitable for the delivery of the unstable macromolecules that ultimately will be the therapeutic compounds with greatest potential benefit.
Direct intracochlear drug delivery, which has been utilized successfully in animals, has significant potential advantages for therapeutic application. The practice of placing drugs of interest within cochlear perilymphatic spaces via a perfusion technique is a method with a long history of successful application. When carefully administered, the technique itself has been shown to have little effect on a variety of gross cochlear and neural potentials as recorded from sites within and near the cochlea. This mode of delivery bypasses the blood-cochlea barrier, allowing drugs to reach their intended targets more directly with lower doses and fewer non-specific actions. Drugs are largely unaltered by metabolic changes that inevitably occur with other routes of administration. Drugs perfused into the perilymph compartment of scala tympani have ready access to the hair cells and synaptic regions of hair cells, a view supported by investigations in which various stains demonstrated ready access to structures within the organ of Corti when introduced via the scala tympani perilymph compartment. Additionally, a comparison of the concentrations of cholinergic antagonists required to block the cochlear efferents in vivo and those effective at in vitro isolated outer hair cells shows remarkable close agreement.
Thus, in order to treat ear disorders, it may often be necessary to deliver therapeutic agents to various ear tissues in a controlled, safe, and efficient manner. For example, a variety of structures have been developed which are capable of delivering/administering therapeutic agents into the external auditory canal of the outer ear. U.S. Pat. No. 4,034,759 to Haerr discloses a hollow, cylindrical tube manufactured of sponge material, e.g. dehydrated cellulose, which is inserted into the external auditory canal of a patient. When liquid medicines are placed in contact with the tube, it correspondingly expands against the walls of the auditory canal. As a result, accidental removal of the tube is prevented. Furthermore, medicine materials absorbed by the tube are maintained in contact with the walls of the external auditory canal for treatment purposes.
However, as mentioned above, the delivery of therapeutic agents in a controlled and effective manner is considerably more difficult with respect to tissue structures of the inner ear (e.g. those portions of the ear surrounded by the otic capsule bone and contained within the temporal bone which is the most dense bone tissue in the entire human body). The same situation exists in connection with tissue materials, which lead into the inner ear (e.g. the round window membrane). Exemplary inner ear tissue structures of primary importance for treatment purposes include but are not limited to the cochlea, the endolymphatic sac/duct, the vestibular labyrinth, and all of the compartments (and connecting tubes) that include these components. Access to these and other inner ear tissue regions is typically achieved through a variety of structures, including but not limited to the round window membrane, the oval window/stapes footplate, the annular ligament, and the otic capsule/temporal bone, all of which shall be considered “middle-inner ear interface tissue structures” as described in greater detail below. Furthermore, as indicated herein, the middle ear shall be defined as the physiological air-containing tissue zone behind the tympanic membrane (e.g. the ear drum) and ahead of the inner ear.
The inner ear tissues listed above are of minimal size and only readily accessible through invasive microsurgical procedures. In order to treat various diseases and conditions associated with inner ear tissues, the delivery of drugs to such structures is often of primary importance. Representative drugs that are typically used to treat inner ear tissues include but are not limited to urea, mannitol, sorbitol, glycerol, lidocaine, xylocalne, epinephrine, immunoglobulins, sodium chloride, steroids, heparin, hyaluronidase, aminoglycoside antibiotics (streptomycin/gentamycin), antioxidants, neurotrophins, nerve growth factors, various therapeutic peptides, and polysaccharides. The treatment of inner ear tissues and/or fluid cavities may involve altering the pressure, volume, electrical activity, and temperature characteristics thereof. Specifically, a precise balance must be maintained with respect to the pressure of various fluids within the inner ear and its associated compartments. Imbalances in the pressure and volume levels of such fluids can cause various problems, including but not limited to conditions known as endolymphatic hydrops, endolymphatic hypertension, perilymphatic hypertension, perilymphatic hydrops, perilymphatic fistula, intracochlear fistula, Meniere's disease, tinnitus, vertigo, hearing loss related to hair cell or ganglion cell damage/malfunction, and ruptures in various membrane structures within the ear.
With respect to existing methods of drug delivery, implantable and externally mounted drug infusers use a “one-way” infusion system where a reservoir empties into the tissue directly or through a catheter. To be pumped along a catheter, however, drugs must have appropriate physical properties. For example, it has been determined that dry compounds, which may be more stable than aqueous ones, cannot be used in a conventional infuser. In another example, it has been determined that highly concentrated compounds may be prohibited because of local reaction at the catheter outlet. Moreover, in the application to inner ear diseases, dosage to the relevant tissues of the cochlea can be difficult or impossible to assess and control by the methods described above, and no device has been provided for programmable long-term delivery, either to the middle ear or inner ear.
Known methods require a relatively complicated mechanism to achieve mixing and circulating flow between reservoir and patient. These more complicated methods include having two tubes entering the patient, rather than just one, or having a two-way pump, two pumps, or a switching valve at the pump.
For example, drugs are delivered to the inner ear by infusing the middle ear and allowing the medication to diffuse through the local tissue and into the inner ear. Alternatively drugs are given systemically (e.g., orally or by injection). For example, U.S. Pat. No. 5,895,372 to Zenner, incorporated by reference herein, discloses an implantable dosaging system that injects drugs into the middle ear using a manually operated pump. As another example, U.S. Pat. No. 6,685,697 to Arenberg et al., incorporated by reference herein, describes a drug delivery unit for controlled delivery of a therapeutic agent to an internal cavity of the ear, particularly to the inner ear, that includes carrier media material containing one or more therapeutic agents therein. The carrier media material is designed to release the therapeutic agents in a controlled manner over time. The drug delivery unit is shaped and sized for placement of at least a portion thereof in the round window niche of a patient.
Alternatively, body fluid is caused to circulate through a drug-containing reservoir via a recirculating system having two tubes—one for inflow and one for outflow between reservoir and patient. For example, U.S. Pat. No. 5,643,207 to Rise, incorporated by reference herein, describes recirculating body fluid through a drug delivery device for drug delivery to the brain. As another example, U.S. Pat. No. 6,561,997 to Weitzel et al., discloses a circuit for extracorporeal treatment of a body fluid.
As another example, one known perfusion technology involves a cochlear implant electrode modified to allow intracochlear drug delivery. In conventional use, the electrode is inserted into the cochlea and used to provide stimulation to the auditory nerve of severely to profoundly hearing impaired individuals. The electrode employed for the drug delivery application, however, contains a removable stylet used for positioning the electrode during insertion. With the stylet removed, the lumen that remains provides the path for drug delivery. The lumen is connected to an osmotic or mechanical pump via a connector and short length of perfusion tubing.
Notably, existing drug-delivery technology is typically not appropriate for long-term programmable infusion into the inner ear. The existing approaches for drug delivery devices include external and implanted infusers, osmotic pumps, and erodible polymer-drug systems. These systems range from passive devices, which have a low level of predictability in their dispense rates, to electronically-controlled rate dispensers and finally to fully programmable infusers. Device volumes range from pill size (e.g., those available from Oculex Pharmaceuticals) to over ten cubic inches, generally depending on their maximum dispense volume and sophistication of control. Though small in volume, erodible polymer and porous membrane systems (e.g., those available from iMMED, Inc.) must typically be implemented to deliver a specific compound, or at best, a set of compounds with similar chemistry and transport properties. They are generally short to medium term delivery devices (less than six weeks) with unalterable, non-constant delivery profiles. The existing osmotic pump-based delivery systems (e.g. those available from Alzet International) are similar in terms of device size and lifetime, and they too are capable only of fixed rate delivery. The various available models trade off device size, lifetime, and delivery rate, depending on the application requirements. Infuser technology has primarily been developed by Medtronic (Minneapolis, Minn.). Devices such as SynchroMed product offer sophisticated control and are effective for treatment for some disorders such as chronic pain. However, because they use macro scale conventionally fabricated pumps, these systems are relatively large. They are practical only when implanted in subcutaneous tissue in the torso.
Emerging microsystems present solution to many previously intractable bioengineering challenges. The extension of micorfabrication methods from integrated circuits to many other applications has spawned microelectromechanical systems (MEMS) devices capable of reproducing the functions of conventional sensors and actuators at a fraction of the size and cost. The resulting miniaturization enables complete systems to be integrated into devices small enough to be implanted in close proximity to the organ to be treated. In the case of drug delivery, complex automated dosing regimens can be programmed into the system or even implemented to respond to sensor input of physiological measurements. Several technologies have emerged that may allow controlled release of drug in dried or lyophilized form from discrete compartments.
And so, as described above, developments in cochlear physiology and molecular biology allow for new and innovative ways of treating and preventing sensorineural hearing loss. It is desirable to implement a safe and reliable mechanism for delivering bioactive compounds directly to the inner ear, e.g., a versatile long-term drug delivery system for the treatment of inner ear disorders that will have broad application and the potential for revolutionizing the treatment of hearing loss.
Thus, it is desirable to provide an implantable long-term drug delivery system for treatment of inner ear disorders and prevention of sensorineural hearing loss, specifically, a versatile device that is capable of delivering multiple simple and complex molecules over long periods of time, with capability to control and regulate the sequence and rate of delivery. Such a device can be useful for treatment of idiopathic and inflammatory conditions affecting the inner ear, including autoimmune inner ear disease, cisplatinum-induced ototoxicity, and possible Meniere's disease. In addition, a wide spectrum of other degenerative inner ear disorders may be amenable to treatment with such a device, including idiopathic, genetically-based, and age-related progressive sensorineural hearing losses.